a) Field of the Invention
The present invention generally relates to a photomask to be used in a photolithography process of semiconductor device manufacture, and to a method of manufacturing a semiconductor device by using such a photomask. The invention specifically relates to a phase shift type photomask and a method of manufacturing a semiconductor device using such a phase shift type photomask.
b) Description of the Related Art
In a Levenson type phase shift photomask, a phase shifter is disposed at one of each pair of adjacent transparent regions. Light transmitted through the transparent region provided with the phase shifter has a phase shift of, for example, 180.degree. relative to light transmitted through the transparent region not provided with the phase shifter. Therefore, the intensity of light exposed on a semiconductor substrate is lowered by light interference at the middle of substrate regions corresponding to each pair of transparent regions. In this manner, a resolution of adjacent bright regions can be improved.
By using a Levenson type phase shift photomask, resolution at the area where the same pattern is regularly repeated can be improved. However, in a practical semiconductor integrated circuit device, not only regularly repeated patterns but also irregular patterns or isolated patterns from nearby patterns are used. There are also connection areas between regular patterns and irregular patterns or isolated patterns.
A sufficient resolution may not be obtained at connection areas between regular patterns and irregular patterns or isolated patterns.